media: bcm2835: unicam: Reduce scope of local function

unicam_start_rx() is not used outside of the unicam module.  Its current
definition produces a compiler warning, that no function prototype
exists.

As the function is only used within the local scope of the module,
convert it to a static function.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>